Reducing a Tropical Cyclone Weak-Intensity Bias in a Global Numerical Weather Prediction System

The operational Canadian Global Deterministic Prediction System suffers from a weak-intensity bias for simulated tropical cyclones.  The presence of this bias is confirmed in progressively simplified experiments using a hierarchical system development technique.  Within a semi-idealized, simplified-physics framework, an unexpected insensitivity to the representation of relevant physical processes leads to investigation of the model's semi-Lagrangian dynamical core.  The root cause of the weak-intensity bias is identified as excessive numerical dissipation caused by substantial off-centering in the two time-level time integration scheme used to solve the governing equations.  Any (semi-)implicit semi-Lagrangian model that employs such off-centering to enhance numerical stability will be afflicted by a  misalignment of the pressure gradient force in strong vortices.  Although the associated drag is maximized in the tropical cyclone eyewall, the impact on storm intensity can be mitigated through an intercomparison-constrained adjustment of the model's temporal discretization. The revised configuration is more sensitive to changes in physical parameterizations and simulated tropical cyclone intensities are improved at each step of increasing experimental complexity. Although some rebalancing of the operational system may be required to adapt to the increased effective resolution, significant reduction of the weak-intensity bias will improve the quality of Canadian guidance for global tropical cyclone forecasting.